I. Field
The present design relates generally to the art of wireless communication systems, and more specifically to reducing variations in signals transmitted in a multi-carrier wireless environment.
II. Description of the Related Art
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so forth. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems.
The 3GPP2 design includes the Evolution-Data Optimized (EV-DO) telecommunications standard, typically employed for broadband Internet access. EV-DO is an evolution of the CDMA2000 standard that supports high data rates and can be deployed with a wireless carrier's voice services. EV-DO standards have included Release 0, Revision A, and Revision B, wherein Rev. B is a multi-carrier evolution of the Rev. A standard. Other competing broadband Internet wireless standards exist, including but not limited to the UMTS standard.
In multi-carrier environments, including but not limited to the EV-DO environment, the peak-to-average ratio (PAR) of a modulated waveform is a metric used to assess transmission performance of a modulated waveform. PAR is sometimes called PAPR, or peak-to-average power ratio, and as used herein, such terms are interchangeable. PAR represents the ratio of the peak value of a signal compared to the average value of the signal. If a modulated signal has a relatively high PAR, the transmitter must perform specific processing to avoid clipping, i.e. reducing the peak values of the transmitted waveform, which is undesirable. Transmission of the full modulated waveform, without reduction, is generally preferred. Additional processing by the transmitter chip to reduce peak values results in a need for additional hardware resources, or in simple terms, requires additional work by the transmitter chip.
Previous systems have addressed issues associated with higher PAR values by employing waveform clipping techniques. Single carrier waveform predictive clipping has been used successfully in wireless communication systems employing single-carrier UMTS technology. In a single carrier arrangement, such predictive clipping is straightforward, in that peak estimation filters are followed by a clipping structure in the transmission sequence. An example of predictive clipping in the single carrier case is provided in FIG. 1, described in detail below.
In a multi-carrier environment, decisions as to where and how to implement predictive clipping functions are more complex in that the predictive clipping components must independently operate in conjunction with the downstream components to effectively and efficiently reduce PAR values in the presence of multiple waveforms. Each carrier stream in a multi-carrier arrangement is independent of the other carrier streams. The overall goal is to reduce the PAR of the aggregate signal, but the peak of any single carrier may or may not contribute to the peak of the aggregate signal. Therefore, simple use of multiple predictive clipping components may not reduce the PAR for all signals transmitted in a multi-carrier environment.
There is therefore a need in the art for techniques and devices that can effectively reduce the overall aggregate PAR of modulated waveforms transmitted in multi-carrier wireless communication networks.